1. Field of the Invention
The present invention relates to a semiconductor process. More particularly, the present invention relates to a method for forming a tunnel oxynitride structure of non-volatile memory.
2. Description of Related Art
Non-volatile memory devices rely on a tunnel oxide structure, which should be sufficiently thin so that carriers can tunnel through the tunnel oxide under a sufficiently high electric field. The quality and stability of the tunnel oxide structure are important issues, since the charge retention capability and other important characteristics of non-volatile memory devices are closely related to these properties.
Many methods provide for improving the quality and stability of a tunnel oxide, which often utilize nitrogen doping of silicon oxide. For example, U.S. Pat. Nos. 5,885,870 and 6,380,033 disclose a method for forming a tunnel oxide structure, where NO or N2O-annealing is performed to add nitrogen to the tunnel oxide structure. The added nitrogen yields benefits such as reducing both the leakage current induced by Fowler-Nordheim stress induced leakage current (FN-SILC) and the threshold voltage (Vg) shift of a non-volatile memory device. However, with such methods, the nitrogen atoms tend to pile up at the bottom interface of the tunnel oxide structure, so that the carrier mobility and the integrity of the periphery gate oxide of the non-volatile memory device are degraded.
Accordingly, it is desirable to manufacture a non-volatile memory device with nitrogen added to the tunneling oxide without piling up all the nitrogen at the oxide interface.